The present invention relates to a method and a device for the high-rate transmission, towards a central station and in a quasi-asynchronous mode, of data coming from a multiplicity of signal acquisition apparatuses which are connected with the central station by common transmission channels.
The method and the device are particularly useful for the sequential transmission, towards a central station, of data coming from a great number of acquisition apparatuses sharing common transmission channels. This is notably the case with transmission systems in offshore seismic prospecting. An often considerable number of seismic receivers, such as hydrophones for example, are distributed at regular intervals all along a supple sheath of very great length or a seismic streamer, consisting of the interconnection of a multiplicity of sections. The seismic streamer is towed behind a ship moving along a seismic profile to be explored, and successive cycles of emission of acoustic signals and of reception of the signals sent back by the subsoil discontinuities in response to the emitted signals are achieved. The signals picked up by the seismic receivers during each cycle are, in recent seismic streamers, collected by acquisition apparatuses arranged in boxes intercalated between the successive streamer sections, sampled, digitized and stored. The different apparatuses are linked by common transmission channels arranged along the streamer, with a central recording station or laboratory located on the ship. One or several channels serve for the transmission of coded command addressed to the different apparatuses by a central laboratory on the ship.
One or several channels are used for the transmission towards the latter of the signals emitted by the different apparatuses in response to the received command. At the end of each emission-reception cycle, the different apparatuses receive commands to sequentially transmit to the laboratory the data that they have stored. Such seismic streamers are described, for example, in French patents 2,471,088 (U.S. Pat. Nos. 4,398,271) or 2,590,684 (U.S. Pat. No. 4,787,069).
The current trend in geophysics is the lengthening of seismic streamers. The correlative increase in the number of receivers allows, as it is well-known, the sharpening of the seismic profiles that can be established from the recordings centralized in the laboratory. Since the number of signals to be transmitted increases, it is necessary to raise the rate of the transmission channels utilized in order to limit as much as possible the intervals between the successive emission and reception cycles. If the objective is to manufacture a seismic streamer that can supply 1,000 to 1,500 distinct recording traces, transmission rates ranging from 30 to 40 Mbits/s are imperative. The most simple method essentially consists in utilizing rapid transmission channels (coaxial cables or optical fibers) and in increasing the frequency of the clock signals punctuating the sequential data transmission. Nevertheless, this results in a rapid increase in the manufacturing cost because the numerous acquisition apparatuses of the streamer should also include electronic components capable of great performance but therefore very costly, especially if the total power consumption is attempted to be limited. This is why the optimizing of the transmission rate of the seismic signals themselves is also sought.
The digitized signals to be transmitted are generally inserted in a frame with a well-developped structure comprising several cells used for identifying the number or signature of the acquisition apparatus in question and the nature of the digitized information included in the frame, so that the reception set in the central laboratory can separate and classify the sequentially transmitted signals.
A well-known method which is applied and described in. the above cited French patent 2,471,088 or U.S. Pat. No. 4,398,271 consists in using a transmission code adapted to the transmission of a clock signal and in fitting each acquisition apparatus in the streamer with emission and reception modules. All the coded digital words in transit on the transmission channels are systematically decoded so that each apparatus can determine if it is actually concerned and, in this case, include into the frame in transit the suitable digitized datum. The positioning of the digital words one after the other is therefore performed in a very rigorous way. At the output of each acquisition apparatus, the digital words are coded again before their application on the transmission channels. The multiplicity of the controls carried out all along the transmission channels has the effect of limiting the centralization speed of the seismic signals. The previous method is therefore very well adapted to transmission rates that are not really high.